1. Field of the Invention
The present invention relates to an apparatus for reproducing a digital signal recorded in a recording medium such as an optical disk, a magnetic disk, magnetic tape or the like.
2. Background of the Invention
Viterbi Algorithm is known as one of methods for decoding a digital signal recorded at a high density in a recording medium with high reliability. In the Viterbi Algorithm, a read signal read from the recording medium is not simply identified as a binary value of 1 or 0 according to a predetermined threshold value, but sample values obtained by sampling read signals are detected as a continuous time-series, and a probable data string is obtained based on this time-series.
A Viterbi decoder assumes state transition in a series of sample values successively supplied from an A/D convertor according to each estimated sample value stored in an estimated value memory (one of the status transitions is called branch, while continuous state transitions are called path), computes a branch-metric indicating probability of a branch as well as a path-metric indicating probability of a path, and decodes a probable data string according to the branch-metric and path-metric.
In the Viterbi decoder as described above, however, if the sample string has its values shifted uniformly, errors are generated in computing a branch-metric as well as a path-metric. Such a generation of error will result in deterioration of the decoding performance.